The invention relates to static dischargers for aircraft.
The build up of electrostatic charges on aircraft in flight and the detrimental effects on the performance of avionics systems on board the aircraft caused by the noisy radio frequency emissions due to the uncontrolled electrical discharge of those electrostatic charges is well known.
Different forms of Static discharger have been proposed all of which aim to allow the dissipation of electrostatic charge which build up on an aircraft in a controlled manner thereby minimising this R.F. noise nuisance.
In general these static dischargers comprise slender elongate rods attached at one end to the skin of the aircraft at a point on the air-frame at which it is anticipated an electrostatic charge would normally be dissipated (e.g. on extremities having high voltage potential gradients such as the trailing edges of wings or tail planes of the aircraft). The high curvature free-ends of the discharger permitting any electrostatic charge which has built up in the aircraft to be discharged at a lower potential voltage than would otherwise be the case. In this way the electrostatic charges which build up on an air-frame whilst the aircraft is flying are dissipated with a lower R.F. noise and therefore less interference with the operation of avionics equipment on board. To enhance the operational life of the static discharger the tips are usually of high resistance material--less likely to be destroyed by the electrostatic discharge taking place therefrom--than a low resistivity material.
More recently static dischargers have been proposed in which the tips of the devices comprise a plurality of fine points (e.g. a mass of carbon fibers) increasing the ease with which the electrostatic charge in the air-frame is dissipated and the operational life of the dischargers.
A difficulty found in operation with known dischargers is that when, as happens in normal use an aircraft extremity is struck by lightning, the lightning strike is attracted to the discharger which is often destroyed together with the surrounding part of the air-frame.
All dischargers known to me include metal parts, having a relatively low specific resistivity of 10.sup.-6 ohmmeter or less (see Kaye and Laby, Tables of Physical and Chemical Constants, 15th Edition, Published by Longmans Inc, New York in 1986--at page 117 et seq) to which tips having a higher specific resistivity are coupled whilst other proposals for dischargers which are, for the main part, of non-metallic materials provide that the discharger is coupled to the skin of the aircraft with use of a coupling--usually of metal--having the noted relatively low specific resistivity or that the discharger is coated with an electrically conductive material--a material having the noted low specific resistivity.
These electrically conductive metal parts are provided in the belief that they ease the operation of the discharger--enhancing the travel of electric charge from the aircraft frame to the discharger tip.
It is believed that the incorporation of these parts of relatively low electrical resistivity in the known dischargers is the cause of lightning being attracted to the dischargers--they form an electrically conductive path for the lightning strike to the aircraft frame.
The incorporation of metal parts and/or coatings in the known dischargers has further disadvantages in respect of corrosion effects of the metal/metal junction of the discharger and air-frame--and in respect of weathering effects.